An electromechanical actuator 100 (FIG. 1) of a valve 110 comprises mechanical means, such as springs 102 and 104, and electromagnetic means, such as electromagnets 106 and 108, for controlling the position of the valve 110 by means of electric signals.
The rod of the valve 110 is applied for this purpose against the rod 112 of a magnetic plate 114 located between the two electromagnets 106 and 108.
When current flows in the coil 109 of the electromagnet 108, the latter is activated and it generates a magnetic action or force, which attracts the magnetic plate 114 and maintains the latter in contact with it.
The simultaneous displacement of the rod 112 now enables the spring 102 to bring the valve 110 into the closed position, the head of the valve 110 coming against its seat 111 and preventing the exchange of gas between the interior and the exterior of the cylinder 117.
Analogously (not shown), when current flows in the coil 107 of the electromagnet 106, the electromagnet 108 being deactivated, it is activated and attracts the plate 114, which comes into contact with it and displaces the rod 112 by means of the spring 104 in such a way that the rod 112 acts on the valve 110 and brings the latter into the open position, the head of the valve being moved away from its seat 111 to permit, for example, the admission or the injection of gas into the cylinder 117.
When the electromechanical actuator 100 is functioning correctly, the valve 110 alternates between the fixed open and closed positions, the so-called switched positions, with transient displacements between these two positions. The open or closed state of a valve will hereinafter be called the “switched state.”
The springs 102 and 104 form an oscillating device characterized by a switching time of the valve with the mobile elements of the actuator 100.
Given the high rigidities k102 and k104 of the springs 102 and 104 and the considerable mass m of the elements being displaced (plate 114, rod 112 and valve 110), the switching time is essentially a function of these rigidities k102 and k104 and of this mass m. Considering that the rigidities k102 and k104 are equal to k, the switching time Δtc is fixed more or less by the square root of the k/m ratio.
In other words, the switching time has low sensitivity to the variations in the current flowing in the coils 107 and 109 of the electromagnets.
The actuator 100 may also be equipped with magnets 118 (electromagnet 108) and 116 (electromagnet 106) intended to reduce the energy necessary for maintaining the plate 114 in a switched position.
Such an electromagnet 106 or 108 with a magnet will hereinafter be called a polarized electromagnet.
The presents invention results from the observation that the optimal switching time for a valve varies depending on the operation of the engine.
For example, a high switching time, using a reduced speed of switching obtained by means of springs of low rigidity, would reduce the impact noises of the plate against the electromagnet and the wear on these components in the case of an engine operating while idling. In fact, such a reduction of the noise would be particularly advantageous for the user of a vehicle while idling because the operating noise of the engine is highly perceptible when the vehicle is stopped.
Inversely, the switching time should be reduced as the speed of the engine increases.
The present invention also results from the observation that the use of a polarized actuator makes it possible to control a magnetic plate with increased sensitivity compared with a nonpolarized actuator, as was shown above [sic-Tr.Ed.] on the basis of FIG. 2.
This FIG. 2 shows the force F (ordinate 200, in N) exerted on a magnetic plate by a deactivated (curve 202) or activated (curve 204) polarized electromagnet and by a nonpolarized electromagnet (curve 206) as a function of the air gap e (abscissa 208, in mm) separating each electromagnet from the plate it controls.
It is seen that the force F exerted by the active nonpolarized electromagnet, i.e., the electromagnet supplied with a current (curve 206) decreases rapidly as a function of the air gap such that this force is relatively weak in the case of an air gap on the order of magnitude of 2 mm.
It should be recalled for this purpose that the force F exerted by a nonpolarized actuator is doubly nonlinear, namely, proportional to the second power of the intensity of the current supplying the electromagnet and inversely proportional to the second power of the air gap.
Inversely, the force exerted by this actuator decreases less rapidly as a function of the air gap in the case of an active polarized electromagnet (curve 204), so that the electromagnet still acts on the plate with an air gap on the order of magnitude of 3 mm.
It shall also be noted that the variation in the force exerted by the polarized electromagnet as a function of the air gap is more linear than the variation in the force exerted by the nonpolarized electromagnet.
Moreover, the reduction in the force exerted by the polarized electromagnet in the case of a small air gap reduces the intensity of the acceleration of the plate and consequently its velocity of impact against the plate, reducing as a consequence the noise generated by the latter.
It is also easier to control the force exerted on the plate with a polarized actuator than with a nonpolarized actuator.
Finally, it is seen that a polarized electromagnet exerts a force on a plate located in the proximity (curve 202) even though it is deactivated, whereas a nonpolarized electromagnet exerts no action in the absence of supply current.